The most memorable open letter I can think of was when Tim O'Reilly published an open letter to Jeff Bezos of Amazon.com discussing Amazon's abusive patents (OneClick ordering -- the IIRC their patented affiliates program). The open letter was also a sort of petition and O'Reilly asked for the signatures of anybody who agreed with him (and yes, I signed). There was a tremendous response (partially due to the opening letter making it onto the /. front page), and so Bezos replied with an open letter of his own. The gist of Bezos' reply was that he agreed the patent system was flawed, but was forced to use the patents (though it was later countered that he chose to use the patent offensively instead of defensively). Bezos' reply caused a bit of commotion and was covered in some of the tech sections of the news saying things like "Bezos speaks in favor of shorter software patents." That sort of thing.

The open letter in this case was effective in raising awareness about problems with the USPTO, but unfortunately, nothing really changed as a result.